Malaria is a vector-borne infectious disease caused by a eukaryotic protist of the genus Plasmodium. It is widespread in tropical and subtropical regions, including parts of the Americas, Asia, and Africa. Each year 350-500 million cases of malaria occur worldwide, and over one million people die, most of them young children (Malaria facts, Center for Disease Control and Prevention. As anti-vector and anti-parasite approaches failed due to resistance to pesticides or resistance to anti-parasite drugs, research efforts began to focus on malaria vaccine development as an effective and inexpensive alternative approach.
Current malaria vaccines are made of attenuated or killed whole pathogens, subunits in the form of purified proteins, peptides, or recombinant DNA that is artificially integrated into living vehicles, such as a viral vector (i.e, adenovirus, vaccinia virus etc.). However, the complex parasitic life cycle has confounded the efforts to develop efficacious vaccines for malaria. Malaria's parasitic life cycle is divided between the mosquito (the insect host) and the human host. While in the human host, it passes through several developmental stages in different organellar environments, including the liver stage and the blood stage. Antigen diversity is a characteristic that must be taken into account in malaria vaccine development, which includes a high degree of developmental stage specificity, antigenic variation and antigen polymorphism. This means different stages of the parasite may require different immune mechanisms for protection.
Vaccine candidates have been identified from each of the parasite's developmental stages.
RTS,S is one candidate entering large scale efficacy trials in Africa, that offers partial protection against infection and clinical disease. Based on the dominant surface protein of the sporozoite (circumsporrozoite protein or CSP), RTS,S is the only subunit vaccine that has consistently demonstrated protection. Recombinant protein vaccines based on other antigens, including thrombospondin related adhesive protein/sporozoite surface protein-2 (TRAP/SSP2), liver stage antigen-1 (LSA1), merozoite surface protein-1 (MSP1), apical membrane antigen-1 (AMA-1), and others have not shown protection to date in experimental sporozoite challenge studies in humans or in field trials in endemic areas. However, in some cases, strain-specific protective effects have been observed.
Faced with limited success, vaccine developers have turned to novel vaccine platforms, such as viral vectors and heterologous prime/boost approaches. With these new approaches, success has been even more modest, with only one of 35 volunteers sterilely protected by a heptavalent poxvirus vaccine called NYVACPf7 and a similar proportion sterilely protected with prime/boost vaccines based on MVA and fowl pox. DNA plasmid vaccines have been particularly disappointing (1).
Some intracellular pathogens such as viruses need strong inflammatory (Th1) responses in order to be cleared. On the other hand, worm parasites and toxin-producing bacteria require anti-inflammatory (Th2) responses for effective neutralization and clearance. Though this follows a general rule still there are infectious agents such as malaria or HIV for which we still do not know the type of immune response required for protection. It is believed that immune evasion induced by the parasite has contributed to the limited success of subunit malaria vaccines.
In order to survive and induce infection, many infectious organisms have developed mechanisms to evade the immune system. One common mechanism is the inhibition of expression of costimulatory ligands on APCs that leads to T cell tolerance to the nominal pathogen [12] Malaria vaccines administered to a subject need to be taken up by immune competent cells, such as antigen-presenting cells (“APCs”) so the immunogenic subunits are processed and degraded into peptides, which are then presented to the T cells in the context of Major Histocompatibility Molecules (MHC). The T cells recognize the MHC-peptide complexes through the T cell receptor (TCR). TCR interaction with MHC-peptide complexes induces early signaling in T cells for activation. However, T cells require two signals to become fully activated. A first signal, which is antigen-specific, is provided through the T cell receptor which interacts with peptide-MHC molecules on the membrane of antigen presenting cells (APC). A second signal, the costimulatory signal, is antigen nonspecific, and is provided by the interaction between costimulatory molecules expressed on the membrane of APC and the T cell. The failure of APCs to provide co-stimulation leads to a state of T cell tolerance to the presented antigens/peptides (11).
Research has been conducted to genetically engineer chimeric proteins with functional properties to allow manipulation of the immune system [3-6, 8-10]. An example of it is shown in U.S. Pat. No. 6,811,785 and US Pub No. 20040234531. However, none of the current approaches present molecule that can provide antigenic as well as costimulatory signals to the immune system. Our genetically engineered chimeric proteins may offer a novel vaccine platform to overcome the T cell tolerance induced by pathogens for treatment or prevention of a human disease.